package com.open.common.persistence.dialect.db;


import com.open.common.persistence.dialect.Dialect;

/**
 * DB2的分页数据库方言实现
 */
public class DB2Dialect implements Dialect {
	@Override
	public boolean supportsLimit() {
		return true;
	}

	private static String getRowNumber(String sql) {
		StringBuilder rownumber = new StringBuilder(50).append("rownumber() over(");

		int orderByIndex = sql.toLowerCase().indexOf("order by");

		if (orderByIndex > 0 && !hasDistinct(sql)) {
			rownumber.append(sql.substring(orderByIndex));
		}

		rownumber.append(") as rownumber_,");

		return rownumber.toString();
	}

	private static boolean hasDistinct(String sql) {
		return sql.toLowerCase().contains("select distinct");
	}

	@Override
	public String getLimitString(String sql, int offset, int limit) {
		return getLimitString(sql, offset, Integer.toString(offset), Integer.toString(limit));
	}

	/**
	 * 将sql变成分页sql语句,提供将offset及limit使用占位符号(placeholder)替换.
	 * 
	 * <pre>
	 * 如mysql
	 * dialect.getLimitString("select * from user", 12, ":offset",0,":limit") 将返回
	 * select * from user limit :offset,:limit
	 * </pre>
	 *
	 * @param sql 实际SQL语句
	 * @param offset 分页开始纪录条数
	 * @param offsetPlaceholder 分页开始纪录条数－占位符号
	 * @param limitPlaceholder 分页纪录条数占位符号
	 * @return 包含占位符的分页sql
	 */
	public String getLimitString(String sql, int offset, String offsetPlaceholder, String limitPlaceholder) {
		int startOfSelect = sql.toLowerCase().indexOf("select");

		StringBuilder pagingSelect = new StringBuilder(sql.length() + 100).append(sql.substring(0, startOfSelect)) // add the comment
		        .append("select * from ( select ") // nest the main query in an outer select
		        .append(getRowNumber(sql)); // add the rownnumber bit into the outer query select list

		if (hasDistinct(sql)) {
			pagingSelect.append(" row_.* from ( ") // add another (inner) nested select
			        .append(sql.substring(startOfSelect)) // add the main query
			        .append(" ) as row_"); // close off the inner nested select
		} else {
			pagingSelect.append(sql.substring(startOfSelect + 6)); // add the main query
		}

		pagingSelect.append(" ) as temp_ where rownumber_ ");

		// add the restriction to the outer select
		if (offset > 0) {
			// int end = offset + limit;
			String endString = offsetPlaceholder + "+" + limitPlaceholder;
			pagingSelect.append("between ").append(offsetPlaceholder).append("+1 and ").append(endString);
		} else {
			pagingSelect.append("<= ").append(limitPlaceholder);
		}

		return pagingSelect.toString();
	}
}
